As it happens, Kevin, I’ve written about that point in the past, so I’ll just quote myself:

Parents pay the costs of raising the children who will one day pay for everyone’s Medicare and Social Security. Childless people thus free-ride on parents’ financial sacrifices. A large child credit is a way of offsetting the federal government’s bias against having children. It’s a move toward governmental neutrality, not social engineering.

The last time I made this point, I got a few rather bitter e-mails from childless conservatives who countered that they already subsidize other people’s children because they pay for public education. But that’s a small part of the cost of raising children. Besides, those e-mailers didn’t pay for their own primary schooling. They want the benefit of having gotten an education without the burden of ever paying for one: another free ride. Asking childless people to help pay for the education of the next generation is already a fair trade. There’s no bias in favor of children for the tax code to offset.